The present invention relates to a taper machining device in a wire-cut electrical discharge apparatus. More particularly, the invention relates to the taper machining device in which a wire electrode is obliquely suspended and has upper and lower bending points, one of the upper and lower bending points serving as a reference vertex and remaining one of the lower and upper bending points being relatively movable within an imaginary conical body having a predetermined apical angle and whose vertex is given by the one of the upper and lower bending points.
In a conventional wire electrical discharge machining apparatus, overrunning or overstroke of a working table is detected by limit switches provided at maximum and allowable moving ends of movable shafts connected to the working table which shafts move the latter in X and Y directions in a horizontal plane. Upon actuation of the limit switches, overrunning or overstoke of the table can be detected.
In a taper machining by means of the wire electrical disharge machining apparatus, a wire electrode is slantingly supported by upper and lower wire electrode guides (hereinafter simply referred to as "wire guide"). One of the wire guides is stationarily supported, whereas the other wire guide is provided movable in a horizontal plane, so that orientation of the wire electrode is changed for performing taper machining. In this machining, overstroke or overrunning of the other wire guide is detected by a limit switch provided at an allowable distal moving end portion.
A vertical distance between the upper and lower wire guides is varried depending on a thickness of a workpiece to be machined. Therefore, as shown in FIG. 5(a), in case of the oblique orientation of the wire electrode 11 for machining the workpiece, even if relative displacement U.sub.X of the upper wire guide 12a.sub.1 and the lower wire guide 13a is equal to that of the upper wire guide 12a.sub.2 and the lower wire guide 13a, inclination angle .theta..sub.1 relative to a vertical line L of the wire 11(1) is much different from that of the angle .theta..sub.2 of the wire 11(2), since the vertical distance H.sub.1 between the upper and lower wire guides 12a.sub.1 and 13a is much different from the distance H.sub.2 between the upper and lower wire guides 12a.sub.2 and 13a.
If the overstroke of the wire guide is intended to be detected only by relying on the relative horizontal displacement of the wire guides, the wire electrode 11 may be excessively inclined in excess of a tolerable inclination in spite of the fact that the overstroke of the wire guide has not yet been detected. As a result, interference may occur between the wire electrode mass and nozzles 24(1), 24(2) and 25 which eject machining liquid.
Further, the one of the upper and lower wire guides is provided movable, for example, the upper wire guide is movable in a horizontal plane defined by U and V axes extending in parallel with X and Y axis along which a working table moves. With the structure, if overstroke is intended to be detected by providing limit switches at distal moving end portions of the U and V axes, overstroke may not be accurately detected at hatching zones shown in FIG. 5(b). More specifically, even though the wire electrode line L.sub.2 has an inclination angle exceeding the tolerable angle .alpha., in light of a three dimensional aspect, such over-inclination cannot be detected in the U and V axes since the line L.sub.2 is projected as if it is positioned within the tolerable inclination angle (for example, projection image of the line L.sub.2 (U.sub.3, V.sub.2) becomes coincident with the projection image of the line L.sub.3 (U.sub.3, V.sub.3) in U axis).
Further, at the time of the electrical discharge, the wire electrode is suspended by the upper and lower wire guides under tension, and is pulledly drawn out in slide contact with the wire guides. Therefore, frictional force is generated at the sliding portion. If the inclination angle of the wire electrode becomes large, the frictional force is also increased, to thereby increase tension applied to the wire. Consequently, the wire electrode may be cut.
To avoid this drawback, the inclination angle of the wire electrode must be within a tolerable range for performing the taper machining. However, vertical distance between the upper and lower wire guides must be changed in accordance with the thickness of the workpiece as described above. Accordingly, for the detection of the overstroke, resetting is required to determine the maximum and allowable moving end position of the wire guide taking the tolerable inclination of the wire electrode into consideration.
Furthermore, in case of the wire-cut machining for producing a product having intricate configuration by moving the working table mounting the workpiece thereon in one moving locus and by moving the wire guide in another moving locus, it would be almost impossible to suitably determine the allowable maximum moving end position in order to detect the overstroke of the wire guide.